1. Field of the Invention
The embodiments discussed herein relate to a semiconductor device such as an insulated gate bipolar transistor (IGBT) of a trench gate structure.
2. Description of the Related Art
In the power electronics field, it is desirable for IGBTs used as switching devices to have low switching loss (turn OFF loss, turn ON loss), which is transient loss, and low saturation voltage (ON voltage) related to steady-state loss, in addition to high switching capability (resistance to latch up). Therefore, improvements to achieve such characteristics are advancing.
Mainstream IGBTs frequently use a striped trench gate structure. A structure that reduces the ON voltage by an intentional expansion of a lower portion (bottom portion) of the striped trench gate structure has been proposed (for example, refer to Japanese Patent Application Laid-Open Publication No. 2013-84922). FIG. 37 is a cross-sectional view of a main portion of a conventional IGBT 800 in which the lower portion of the trench gate structure has been expanded. In FIG. 37, reference sign 51 represents a p-type collector layer; reference sign 52 represents an n-type buffer layer; reference sign 53 represents an n-type drift layer; reference sign 54 represents a p-type base layer; reference sign 55 represents a trench; reference sign 56 represents a gate insulating film; reference sign 57 represents an n-type emitter layer; reference sign 58 represents a p-type contact layer; reference sign 59 represents a gate electrode; and reference sign 60 represents a collector electrode.
When across the entire region of the n-type drift layer 53, the lower portion of each trench 55 is expanded, lower portions of the n-type drift layer 53 between adjacent the trenches 55 form narrow stripes. The technique described in Japanese Patent Application Laid-Open Publication No. 2013-84922 adopts this structure and thereby, suppresses the amount of holes drawn from the p-type contact layer 58 through the p-type base layer 54 disposed on the n-type drift layer 53 and causes holes to accumulate in the n-type drift layer 53, which is an upper portion connected to the channel. Consequently, the amount of electrons attracted by the accumulated holes and injected into the n-type drift layer 53 through the channel increase, whereby the ON voltage decreases. Increasing the amount of electrons injected from the channel by an accumulation of holes in this manner is called the injection enhancement (IE) effect.
Another conventional technology includes L-shaped trench gates formed in a vertical direction with respect to a first principal surface of the n−-type layer, from a surface of a p-type base layer to a position inside an n−-type layer (drift layer). The L-shaped trench gates have a bottom extended on one side for a predetermined length, in a horizontal direction with respect to the first principal surface of the n−-type layer. Predetermined adjacent L-shaped trench gates are disposed with the bottoms expanding in opposing directions to face each other and are spaced at an interval that is less than an interval between portions formed in vertical direction with respect to the first principal surface of the n−-type layer, thereby enabling reduction of the ON voltage and loss (for example, refer to Japanese Patent Application Laid-Open Publication No. 2008-60138).
According to another conventional technique, a semiconductor device has an n+-type emitter region connected to an emitter electrode, a p−-type body region surrounding the emitter region and connected to the emitter electrode, an n−-type drift region contacting the body region and isolated from the emitter region by the body region, and a trench gate electrode opposing, via a gate insulating film, the body region isolating the drift region from the emitter region. In the semiconductor, a portion of the trench gate electrode in a longitudinal direction of the trench gate electrode is formed to have a different trench width, whereby the minority carrier concentration in the body region is decreased (for example, refer to Japanese Patent Application Laid-Open Publication No. 2005-150246).